Tires are the one thing you shouldn't skimp on, but a full set of all-terrains is a sting to the wallet. You want tires that handle the dirt and the commute without wearing down prematurely, which usually lands the total around $1000.
Wait until early spring to pull the trigger. That is when manufacturers roll out their best mail-in rebates to get people ready for road trip season. If you time the purchase with these deals, you can easily knock a chunk off the sticker price before you even swipe a card.
The 18-month window is a deadline, not a pass to pay whenever you feel like it. If you still have a balance when that time is up, you will start paying interest on whatever remains, which is where the plan gets expensive.